Conventionally, there have been used projected image display units of a rear projection type, which project an image onto a transmission screen with a projector from the backside (rear side) of the screen opposite to the surface viewed by a viewer (front surface).
FIG. 1 is a diagram showing examples of the projected image display unit. FIG. 1 shows two projected image display units 1 and 2. The projected image display unit 1 is a vertical type, and the projected image display unit 2 is a table type.
Some of such projected image display units are capable of capturing the image of an object on the front surface of a screen 3 with a camera from the rear side of the screen 3.
The image capturing is affected by the contrast ratio of a projector. The contrast ratio of the projector is expressed by the ratio of the brightness of an all white projection to the brightness of an all black projection. When the brightness of a lamp is constant, low contrast means high brightness of black projection, and the transmittance of the lamp at the time of black projection is also high.
Many recent DLP projectors enjoy high contrast, in which the effect of a lamp center light is relatively small. However, the effect of the lamp cannot be ignored even in black projection in conventional projectors with approximate contrast of 50:1 to 1000:1. (See Patent Documents 1 through 6.)
Further, in recent years, this rear-projection-type display has been used in demonstrations in events, conferences, etc., and there is strong demand for the function of displaying a drawn image written onto the front surface side of a screen on the screen in combination with an image projected onto the screen and the function of manipulating an image projected onto the screen by directly touching the front surface of the screen with a hand.
One method that satisfies this demand is to capture the image of an object on the front surface of a screen from its rear surface side with a camera using a CCD solid-state image sensing device and extract necessary information from the captured image.
In general, screens conventionally used for rear-projection-type displays, which give importance to visibility at the time of viewing from the front side, are provided with a Fresnel lens for making uniform the amount of light at the time of viewing the projected light of a projector from the front side and a lenticular lens for improving visibility. However, capturing an image on the screen using them from its rear side results in a distorted image. Further, there is a problem in that while it is necessary for external light from the front side of the screen to be sufficiently transmitted to its rear surface side in order to have good image quality, an increase in the scattering performance of light projected from the rear side decreases the transmission performance.
Therefore, for example, a mask is provided between a projector and a screen, and the mask is caused to be in a transmission state during a projection display period and in a light interruption state or a light scattering state during an image capturing period. (See Patent Document 7.)
Further, a screen is formed by sandwiching liquid crystal between a lenticular lens and a Fresnel lens, and an electric field is applied to the liquid crystal so that the liquid crystal matches the lenticular lens in refractive index so as to cause the screen to be in the light transmission state at the time of image capturing and the liquid crystal does not match the lenticular lens in refractive index so as to cause the screen to be in the light scattering state at the time of displaying. (See Patent Document 8).
However, these techniques make it expensive to manufacture screens, and make it necessary to control screen conditions, a camera, and projection with high accuracy at high speed in order to prevent a user viewing the surface of projection from the front side from feeling oddness, thus complicating system configuration and control.
On the other hand, a region that diffuses light and a region that transmits light from the front surface are provided like a lattice, thereby improving the visibility of a projected image and the quality of an image captured by a camera on the rear side. (See Patent Document 9.)
This technique has the advantage that external light is taken in with efficiency, the visibility of a projected image is prevented from being degraded, and there is no need to switch states during use of the apparatus. However, there are the problems of high technical difficulty and high cost because the diffusion region and the transmission region of the screen should be placed with high accuracy considering displaying a projected image with good quality to a viewer on the front side of the screen and the quality of the image captured from the rear side.
Further, image information and conference image information can be stored or transmitted to a remote place by using a screen selectively set to a scattering state or a transmission state electrically, and projecting an image from the rear side of the screen and capturing the image of original material and participants disposed on the front side of the screen from the rear side of the screen. (Patent Document 10.)
[Patent Document 1] Japanese Laid-Open Patent Application No. 2004-109402
[Patent Document 2] Japanese Laid-Open Patent Application No. 2003-094886
[Patent Document 3] Japanese Laid-Open Patent Application No. 2004-032665
[Patent Document 4] Japanese Laid-Open Patent Application No. 2003-153136
[Patent Document 5] Japanese Laid-Open Patent Application No. 09-326981
[Patent Document 6] Japanese Patent No. 3171955
[Patent Document 7] Japanese Laid-Open Patent Application No. 06-269002
[Patent Document 8] Japanese Patent No. 2940716
[Patent Document 9] Japanese Laid-Open Patent Application No. 2000-101981
[Patent Document 10] Japanese Laid-Open Patent Application No. 2003-94886